It is known that compounds which contain positively-polarized chlorine atoms, such as N-chloro compounds, and compounds which contain negatively-polarized chlorine atoms, such as hydrochloric acid, may react with each other and split off chlorine in the process. These reactions generally require high temperatures or even equimolar quantities of Friedel/Crafts catalysts. Moreover, the reaction is not generally practicable and is often accompanied by side-reactions or secondary reactions so that it is impossible to predict the main reaction product.
It is also known, (DAS No. 1,266,288), to prepare carbonyl diisocyanate by the thermolysis of N-trichloroisocyanuric acid at a temperature of from 200.degree. to 400.degree.C, accompanied by the formation of NCl.sub.3, in accordance with the following equation: ##SPC1##
The reaction is accompanied by side-reactions and is difficult to control due to the simultaneous formation of the highly explosive compound, NCl.sub.3.
Another process for the preparation of CO(NCO).sub.2 is the reaction of difluorophosgene with potassium cyanate in molten LiCl/KCl at temperatures of about 400.degree.C. This method of preparation involves relatively high technical expenditure because of the low conversion rates and because of high reaction temperatures required, (Angew. Chem. 79, 860 (1967)).